In a large skillet, heat the olive oil over medium heat. Add the chopped onion and bell pepper, and sauté until softened, about 5 minutes. Season with salt and pepper.
Place the chicken breasts in the bottom of the crockpot. Sprinkle the taco seasoning over the chicken, then add the salsa, black beans, corn, and sautéed onion and bell pepper.
Cover the crockpot and cook on low for 6-8 hours or on high for 3-4 hours, until the chicken is cooked through and shreds easily with a fork.
Once cooked, shred the chicken in the crockpot using two forks and stir to combine with the other ingredients.
Warm the tortillas in a dry skillet or microwave. Serve the chicken mixture in the tortillas, topped with shredded cheese, sour cream, and chopped cilantro.